A circuit pattern is formed on a surface of a wafer of a semiconductor such as silicon or gallium arsenide by, for example, etching or liftoff method. Subsequently, the wafer having a circuit pattern formed thereon is subjected to polishing the back of the wafer by means of, for example, a grinder in the state that its patterned surface is generally covered with a pressure sensitive adhesive sheet. The first object of polishing the back of the patterned wafer is to remove any oxide coating which may occur on the back of the wafer at the etching step. The second object of polishing the wafer back is to regulate the thickness of the wafer having the circuit pattern formed thereon.
The polishing of the back of the wafer having the circuit pattern formed on its surface is conducted while washing the wafer back with purified water in order to remove dust raised by polishing and heat generated during the polishing. Before the polishing of the wafer back is conducted, the pressure sensitive adhesive sheet (surface protective sheet) is adhered to the wafer surface so that the circuit pattern formed on the wafer surface is protected from polishing dust and cutting water.
This type of pressure sensitive adhesive sheet is, for example:
a hydrophilic type such as the pressure sensitive adhesive sheet of Japanese Patent Laid-open Publication No. 62(1987)-101678 using a pressure sensitive adhesive containing a nonionic surfactant or the pressure sensitive adhesive sheet of Japanese Patent Publication No. 5(1993)-77284 using a water swellable pressure sensitive adhesive, which, after the completion of the polishing, is directly peeled so that any stain on the wafer surface caused by the pressure sensitive adhesive components can easily be removed by washing with purified water; or PA1 an energy beam curable one such as the pressure sensitive adhesive sheet disclosed in Japanese Patent Laid-open Publication No. 8(1996)-27239, which, after the completion of back polishing, is irradiated with energy beams to thereby extremely reduce the adhesive strength to the wafer so that traces of the pressure sensitive adhesive do not remain upon peeling of the pressure sensitive adhesive sheet. PA1 a carboxyl group containing copolymer obtained by polymerizing a carboxyl group containing a first polymerizable monomer (A1) and a second monomer (A2) that is copolymerizable with the carboxyl group containing polymerizable monomer according to solution polymerization, a neutralizer (A3), and a crosslinking agent (A4). PA1 adhering the above pressure sensitive adhesive sheet to a patterned wafer surface, and PA1 polishing the back of the wafer while feeding water to the wafer back.
Although the above two types of pressure sensitive adhesive sheets for wafer back polishing have appropriately been employed in view of the respective characteristics and in conformity with encountered conditions, an energy beam curable pressure sensitive adhesive sheet which exhibits low peeling resistance at the time of peeling of pressure sensitive adhesive sheet and which does not break the wafer at the time of peeling be in demanded in the future due to the increased wafer diameter and reduced thickness of the wafer.
However, this energy beam curable pressure sensitive adhesive is not water washable. Thus, the problem would be encountered such that, when a residual pressure sensitive adhesive remains on a wafer surface for some reason (e.g., shortage in quantity of energy beams applied at the irradiation therewith, abnormally high temperature caused by excess irradiation, energy beam irradiation deficiency attributed to adhesion of cutting dust raised at the time of wafer back polishing onto the surface of the sheet, etc.), the residual pressure sensitive adhesive cannot be removed by water washing. Thus, when a conventional energy beam curable pressure sensitive adhesive remains on a wafer surface, it is common practice to conduct washing with an organic solvent. This, however, requires improvement from the viewpoint of, for example, environmental conservation.
The present invention provides a pressure sensitive adhesive composition suitable for use in a wafer surface protective sheet which, at the time of polishing of a wafer back, is used to protect the circuit pattern formed on the wafer surface from cutting dust, etc.
The present invention further provides a pressure sensitive adhesive composition suitable for use in a wafer surface protective sheet in which, even if a pressure sensitive adhesive remains on a wafer surface upon peeling of the pressure sensitive adhesive sheet, the residual pressure sensitive adhesive can easily be removed from the wafer surface by washing with water.